Transport of Carbon Tetrachloride in Karst Aquifer

摘要

Karst water in Ordovician and Cambrian Limestone aquifers is a main source of water supply in Xuzhou City, yielding up to 35,000 m~3 per day. According to monitoring data in November 2000, it has been contaminated with carbon tetrachloride(CT) in Qiligou water-bearing basin. The pollution source is Shanpu Pesticide Plant Which produced a kind of pesticide used carbon tetrachloride as solvent, and this plant is located at hill slope in recharge area of the karst aquifer. CT concentration of wastewater of this plant is from 281.0 μg/L to 2584.3 μg/L. Till May 2001, CT was found in 53 wells (contaminated area is 17.3 km~2) and the highest CT concentration in groundwater was up to 3909.2 μg/L. According to CT concentration, the contaminated area can be divided into three sub-areas: pollution source sub-area that the highest CT concentration in initial period and at present is 3909.2 and 1891.5 μg/L respectively; sub-area of artificial discharge center which the highest CT concentration is 504.5 μg/L at present and transition sub-area between them. The most seriously polluted center has transported 2 km to discharge direction in past three years, and the highest CT concentration in well has been reduced from 3909.2 μg/L to 1891.5 μg/L. Monitoring data of 50 wells in past three years has shown that CT concentration in karst aquifer was controlled synthetically by precipitation, piezometric level and the depth of aquifer. Carbon tetrachloride as a dense nonaqueous phase liquid (DNAPL) with slight dissolubility mainly transports along the bottom of the karst aquifer, therefore, CT concentration in well depends on the depth of the well, usually relative lower occurs in the wells with the depth less than 150 m and higher with more than 150 m.